Precision welders are widely used in electronic fabrication operations. In the assembly of modern electronic components, extremely fine wires are connected from electronic components to circuit interconnection points on, for example, printed circuit boards. It is important to be able to control and deliver a precise amount of energy to each weld site in order to obtain a truly acceptable, electrically sound interconnection of these wires to their interconnection points.
It is equally well known that the physical parameters of each weld job have a tendency to vary due to a number of factors, including the presence of dirt or dust particles, oxidation and finish of the wire surface or the surface of the interconnection point, and variations in wire diameter and in the electrical resistance of the wire and the connection point. Such variations in the weld site conditions, of necessity means that the precise amount of energy needed is likely to vary from site to site and job to job.
In order to obtain uniform, repeatable, metallurgically acceptable welds, therefore, a precision welder must be operated in a way that recognizes that there will be differences in resistance from weld site to weld site. Typically, the site to site resistance may vary from 50% to 200% from the normal. These variations in electrical resistance in turn produce variations in the amount of energy delivered to the weld site. Most weld monitors depend upon measuring the delivered energy and, in some form, deciding on the basis of this measurement whether a weld will be good or bad. These energy measurements, in engineering terms, are expressed as peak volts, peak amps, volt-seconds, amp-seconds, and watt-seconds.
All these values are useful measures of energy, but if the weld resistance changes, the energy delivered to the weld site may not change appreciably. For instance, when the resistance increases, the voltage will increase, but the current will decrease. Thus, the product of voltage and current (watts or watt-seconds) will only show a change smaller than the change in either volts or amps.
In addition to the above factors, many other factors also affect weld resistance. Among these parameters are changes in the materials being welded, the weld temperature, plating thickness, electrode pressure, electrode diameter and surface area. Each change in such parameters produces a change in weld resistance. It would be extremely desirable therefore to be able to monitor resistance change, but measurement of resistance using electronic circuit means is difficult to achieve, involving as it does the arithmetic operation of division, in this case, division of the voltage drop across the weld by the current through the weld. In contrast, the arithmetic operations of addition, subtraction and multiplication of voltages and currents ar relatively easy to achieve electronically.
Variations in resistance from one weld site to the next will cause variation in the magnitude of the instantaneous electrical power delivered to the weld sites if the same amount of instantaneous drive current is delivered to the weld sites. Producing a strong, metallurgically acceptable weld depends upon adhering to a weld schedule to deliver the right amount of energy within the right amount of time. Failure to deliver the proper amount of energy to the weld sites results in welds which are weak and unacceptable.
In general, precision resistance welders include adjustment controls to enable an operator to select the desired setting for the magnitude and duration of the energy that the spot welder will supply in a weld site. In a production setting, the actual weld resistance is not measured each time.
A method of measuring weld resistance is to pass a known d.c. test current through the series path defined by the welding electrodes of the spot welder and the weld site, and measure the resulting voltage that is developed across the welding electrodes. However, this technique may be insufficiently sensitive since voltage changes tend to produce opposite changes in current which, when translated into power changes result in little or no significant change in the energy delivered to the weld, even though the actual resistance of the weld may have changed significantly from its normal value, e.g. an increase to 30 milliohms or a decrease to 7.5 milliohms from a nominal value of 15 milliohms.